OvmfPkg/XenBusDxe: Add XenStore function into the XenBus protocol

OvmfPkg/XenBusDxe: Add XenStore function into the XenBus protocol

Change in V3:
- Have XenStoreWaitWatch/XenBusWaitForWatch return a XENSTORE_STATUS
  instead of VOID.
- Add description of the introducted member of the protocol.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16269 6f19259b-4bc3-4df7-8a09-765794883524